[Python] why numpy's nextafter(0,inf) will raise err when flag under(flow) enabled?

from numpy import nextafter,seterr
print(nextafter(0.,1.)) #5e-324
seterr(under="raise")
print(nextafter(0.,1.)) #FloatingPointError: underflow encountered in nextafter


Why is this code truggering the error below ?

FloatingPointError: underflow encountered in nextafter.


According to my tests, similar code works in C.

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